Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

Planet-A (renamed Suisei, the Japanese word for "comet," after launch), was the second of two Japanese missions to Comet Halley during the 1986 encounter. The cylindrical spacecraft was launched directly on a deep-space trajectory without entering intermediate-Earth orbit.

The main payload of the spacecraft was an ultraviolet-based imaging system that could study the huge hydrogen corona around the comet. The spacecraft's measurements as the hydrogen cloud grew helped to determine that the gas was released from the break-up of water vapor at a rate of no more than 60 metric tons per second.

After a course correction on 14 November 1985, Suisei flew within 152,400 kilometers of the comet's nucleus on 8 March 1986 at 13:06 UT, returning ultraviolet images of the 20-million-kilometer-diameter hydrogen-gas coma. Even at that relatively large distance from the comet, the spacecraft was hit by at least two dust particles, each 1 millimeter in diameter.

After the Halley Encounter, in 1987, ISAS decided to send the spacecraft through an elaborate trajectory for an encounter with Comet Giacobini-Zinner on 24 November 1998, thirteen years after launch. Suisei performed a series of trajectory corrections between 5 and 10 April 1987 in order to send it on a gravity-assist around Earth on 20 August 1992 at a range of 60,000 kilometers.

Unfortunately, hydrazine for further corrections was depleted by 22 February 1991. The planned encounter with Giacobini-Zinner (as well as a far-distance flyby of Comet Tempel-Tuttle) had to be canceled on 28 February 1998. The cancellation formally ended the mission.

Key Dates

18 Aug 1985: Launch

8 Mar 1986: Comet Halley Flyby

Status: Successful

Fast Facts

Suisei means comet in Japanese. It was also the name of a Japanese World War II fighter (right).

Even though it kepts it distance from comet Halley, Suisei was still hit by dust particles.